REFLECTOR:Re: NPG+

Mon, 19 Apr 2004 09:10:35 -0500

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Weeeell, almost.  Actually, water cooling systems are usually not
pressurized.  To be pressurized the coolant temp would need to be above
it's atmospheric boiling point (212-215F).  The thermostat controls the
temp below that.  The reason you have pressure when you open the cap is
because once the engine is off, the radiator can not cool, but the
engine is still hot and heats the coolant above the boiling point which
results in temporary pressure buildup.
If the cooling system were pressurized and pressure was lost, there
would be a temporary surge (possibly a steam layer) in the boiling rate
and then normal boiling would resume.  The only way to stop nucleate
boiling is to coat the jacket surface with oil.  In that case the
coolant would superheat (about 20F above the normal boiling point) and
boil spontaneously and the cooling would be about the same.
Thinking about it further, the coolant temp is probably a good estimate
of the CHT, which means that a hotter coolant temp would be beneficial
as Bill pointed out.

John

Chuck Jensen wrote:

>  John,I think I've got it straight.  You're right (which is probably
> not a surprise to you), in that boiling does go on in a pressurized
> H2O cooling system.  Nucleate boiling forms right at the metal surface
> and when the bubble moves away from the metal, they immediately
> collapse.  This is a very effective heat transfer mechanism.  If
> pressure on the system is lost, then a steam layer will form on the
> metal, heat transfer will suffer greviously and it would be a good
> idea to have a landing site in mind.  So, the NPG+ cooled engine
> probably runs hotter since it won't support nucleate boiling for good
> heat transfer.  Is that good?Chuck

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Weeeell, almost.&nbsp; Actually, water cooling systems are usually not
pressurized.&nbsp; To be pressurized the coolant temp would need to be
above it's atmospheric boiling point (212-215F).&nbsp; The thermostat controls
the temp below that.&nbsp; The reason you have pressure when you open the
cap is because once the engine is off, the radiator can not cool, but the
engine is still hot and heats the coolant above the boiling point which
results in temporary pressure buildup.
 If the cooling system were pressurized and pressure was lost, there
would be a temporary surge (possibly a steam layer) in the boiling rate
and then normal boiling would resume.&nbsp; The only way to stop nucleate
boiling is to coat the jacket surface with oil.&nbsp; In that case the
coolant would superheat (about 20F above the normal boiling point) and
boil spontaneously and the cooling would be about the same.
 Thinking about it further, the coolant temp is probably a good estimate
of the CHT, which means that a hotter coolant temp would be beneficial
as Bill pointed out.
John
Chuck Jensen wrote:
<blockquote TYPE=CITE>&nbsp;John,I
think I've got it straight.&nbsp; You're right (which is probably not a
surprise to you), in that boiling does go on in a pressurized H2O cooling
system.&nbsp; Nucleate boiling forms right at the metal surface and when
the bubble moves away from the metal, they immediately collapse.&nbsp;
This is a very effective heat transfer mechanism.&nbsp; If pressure on
the system is lost, then a steam layer will form on the metal, heat transfer
will suffer greviously and it would be a good idea to have a landing site
in mind.&nbsp; So, the NPG+ cooled engine probably runs hotter since it
won't support nucleate boiling for good heat transfer.&nbsp; Is that good?Chuck</blockquote>

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